Flagella-Driven Chemotaxis Towards Exudate Components Is an Important Trait for Tomato Root Colonization by Pseudomonas fluorescens

Abstract

Motility is a major trait for competitive tomato root-tip colonization by Pseudomonas fluorescens. To test the hypothesis that this role of motility is based on chemotaxis toward exudate components, cheA mutants that were defective in flagella-driven chemotaxis but retained motility were constructed in four P. fluorescens strains. After inoculation of seedlings with a 1:1 mixture of wild-type and nonmotile mutants all mutants had a strongly reduced competitive root colonizing ability after 7 days of plant growth, both in a gnotobiotic sand system as well as in nonsterile potting soil. The differences were significant on all root parts and increased from root base to root tip. Significant differences at the root tip could already be detected after 2 to 3 days. These experiments show that chemotaxis is an important competitive colonization trait. The best competitive root-tip colonizer, strain WCS365, was tested for chemotaxis toward tomato root exudate and its major identified components. A chemotactic response was detected toward root exudate, some organic acids, and some amino acids from this exudate but not toward its sugars. Comparison of the minimal concentrations required for a chemotactic response with concentrations estimated for exudates suggested that malic acid and citric acid are among major chemo-attractants for P. fluorescens WCS365 cells in the tomato rhizosphere.

Transcriptome profiling of bacterial responses to root exudates identifies genes involved in microbe-plant interactions

“…1). Again, this finding could reflect differences in levels of key attractants in the two exudates that could conceivably play a role in host selection because chemotaxis plays an important role in successful colonization of the rhizosphere (21,(24)(25)(26)(27)(28). Some genes encoding proteins with functions previously implicated in microbial-plant interactions were down-regulated in response to both root exudates (Fig.…”

Rhizosphere microbiome assemblage is affected by plant development

“…Interestingly, we see that the two-component system, chemotaxis family, response regulator involved in bacterial chemotaxis is significantly expressed late in plant development and its expression positively correlates with the root exudates glycine and xylitol (Supplementary Tables S3 and S4). Interestingly, PGPRs and endophytic bacteria show chemotaxis towards glycine (Gaworzewska and Carlile, 1982;de Weert et al, 2002;Gupta Sood, 2003), while the non-symbiotic nitrogen-fixing bacteria Azotobacter vinelandii shows chemotactic behavior towards xylitol (Haneline et al, 1991). This provides an additional mechanism for the plants ability to manipulate and orchestrate the rhizosphere microbiome.…”

“…Evidence demonstrating the close ties root exudates have on the microbial composition of the rhizosphere is mounting (Broeckling et al, 2008;Badri et al, , 2013aMicallef et al, 2009b;Chaparro et al, 2012Chaparro et al, , 2013, whereby many chemicals present in root exudates act as substrates, chemotactic or signaling molecules to orchestrate changes in microbial composition (Shaw, 1991;de Weert et al, 2002;Jain and Nainawatee, 2002;Horiuchi et al, 2005;Bais et al, 2006;Badri and Vivanco, 2009;Neal et al, 2012;Badri et al, 2013a). Recently, it was reported that the composition of Arabidopsis root exudates change following a plant developmental gradient (Chaparro et al, 2013).…”

Isolation and selection of fluorescent pseudomonads based on multiple plant growth promotion traits and siderotyping

“…de Weert et al (2002) established that motility in Pseudomonas is a major trait for root colonization, and the organic acids such as malic acid and citric acid are among major chemo-attractants for P. fluorescens in the tomato rhizosphere. This study is supported by Choi et al (2006) that biofilm formation and swarming motility may contribute to bacterial colonization and stable maintenance of antifungal compounds on plant surface.…”